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Shore Hardness

12/13/2010 12:20 PM

Hallo all,

I'll try to give you all the details, and if I'm missing something tell me.

I know there is different types of shore scale (A,D),

I need to design some sort of seal that made of EPDM. I know the EPDM come in different hardness (shore A, D).

I found in wikipedia the Relation between Shore hardness and elastic modulus but it give me different results from the table that I found.

My question is, how do I calculate how much the EPDM shrink if I know the shore num?

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#1

Re: shore hardness

12/13/2010 12:34 PM

I don't believe that the Shore hardness # should enter into your shrinkage calculations at all. There are too many other factors involved when molding the elastomer.

You should select the desired hardness and get information from the supplier of the material regarding mold shrinkage.

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#2

Re: Shore Hardness

12/13/2010 1:41 PM

1- the table is NOT for elastomer so that do not be surprised. The durometers are different.

2- why do you need the E for EPDM ? do you want to make a FEA simulation ? If it is the case you MUST consider the non linearity and the temperature effect which is VERY important.

3- If you want to use a standard seal look at the application and make the choice according to recommendations.

4- What kind of shrinkage do you think of ? If when for the form then you do not need it, if you want to compute (simulate) compression then you need it.

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#3

Re: Shore Hardness

12/13/2010 3:55 PM

I want to seal between two platforms. the surface finish is not very straight so I want the EPDM to close all the gaps.

I'll still don't know the dimension if the highest "bump" (I don't know the word for it in English), I just wanna know how do I know how much the EPDM shrink while applying pressure on it according to the shore number (if it possible) and if not, how do i do it.

I do need the E, the choice of material wasn't mine… (Not enough rank yet).

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#4
In reply to #3

Re: Shore Hardness

12/13/2010 4:29 PM

Compressive modulus of elasticity is what you want, not shrinkage.

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#5
In reply to #4

Re: Shore Hardness

12/13/2010 5:15 PM

this was the formula i was planning to use after i get the Young's modulus from the shore number, but how do I get the Young's modulus from the shore number?

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#6
In reply to #5

Re: Shore Hardness

12/13/2010 6:47 PM

Forget the Shore number. It has no bearing on compression. It is a measurement of penetration resistance.

Compression!

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#7
In reply to #6

Re: Shore Hardness

12/14/2010 3:28 AM

Yes, BUT there is a relationship between "intrinsic material stiffness" which measured by the Young modulus and the "penetration" used by the Shore durometer. I shall have a look I had such an indication but I do not know now where it is. I think that with google 's help it could be possible to find even more modern documentation.

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#10
In reply to #7

Re: Shore Hardness

12/14/2010 7:55 AM

Correct. But the OP seems to be focused on Shore hardness when he should be thinking about Young's modulus.

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#8
In reply to #6

Re: Shore Hardness

12/14/2010 3:30 AM

ok.

So I need to calculate how much is the Young's modulus I need from the displacement equation and ask the manufacture to give me the material with this number?

Or Is there any table or any other accessorize that can help me with it?

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#9
In reply to #8

Re: Shore Hardness

12/14/2010 3:42 AM

the equation from wikipedia is the equation?

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#13
In reply to #9

Re: Shore Hardness

12/14/2010 1:14 PM

Yes but it is only an approximation the real value can differ. There is more difficult problem in the computation the fact that elastomer's have a Poisson' s coefficient near to 0.5 (do not change volume under compressive strain) and that the "apparent" module depends on the friction between seal and surfaces. The seal will be in the assembly "stiffer" since it cannot expand due to friction. The magnitude of this "stiffness complement" depends on the seal geometry. This is the reason why in the design of elastomer damper the stiffness is computed considering a form factor.

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#11

Re: Shore Hardness

12/14/2010 11:40 AM

Normally you can't tell the shrink by the hardness. Shrink is usually associated with the Polymer family and is really dependant on the compound ingredients and the method of processing (ie;plasticisor content, cure cycle and post cure). Think of it like this, the more volitile additives in the compound, the more to outgas during cure, and the greater the shrink. Higher processing temperatures and post curing may add to this effect. Some Silicone compounds may shrink up to 3.2%, while your Nitrile & EPDM compounds typically shrink at APPROXIMATELY 1.8% (thats .018 of an inch per linear inch of seal). Other processing factors include the molding method, ie; compression, which way is the grain of the preform running? The only real way to determin the real shrink factor is to run a study on the compound/process and obtain results by measuring the specimens.

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#12
In reply to #11

Re: Shore Hardness

12/14/2010 1:00 PM

Guest,

I believe that Avisar has confused the terms here. I get the impression that he may be new to elastomers. While he is referring to shrinkage, I believe that he is really concerned about compression in service and is incorrectly using the terms.

Shrinkage, as you have indicated, is more of a processing issue and once the part is molded it is no longer a factor, except in the sense of thermal shrinkage, which can be fairly high with elastomers.

I feel that he is really trying to predict the amount of compression the part will see when put into service.

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#14
In reply to #12

Re: Shore Hardness

12/15/2010 6:40 AM

Indeed i do.

The approximation equation is fine with me (I think) coz i'm planning to use some safety factor for this.

The seal geometry is a square shape and it suppose seal a glass surface and a ULTEM surface.

Or maybe I'm asking the wrong questions and I need to know something or consider something else while approaching this problem or I'm just confused?

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